The present invention relates in general to code division multiple access (CDMA) communications systems and, more particularly, to an improved method of interference cancellation for CDMA communications systems using M-ary orthogonal modulation.
Multiple access communications techniques enable multiple users, such as mobile telephones, to share the same path, for example a radio channel, to communicate to one receiver or base station at the same time. Examples of multiple access techniques include frequency division multiple access (FDMA) wherein different users are assigned to different frequency bands of the channel, time division multiple access (TDMA) wherein different users are assigned to non-overlapping time slots of the channel, and code division multiple access (CDMA). In CDMA, different users are assigned unique spreading codes, commonly pseudorandom noise (PN) codes, which are high bandwidth bitstreams used to multiply a respective baseband signal before transmission. Multiplying a baseband signal by a spreading code increases the bandwidth of the signal by a factor known as the spreading gain to spread the baseband signal across the channel.
Upon receipt at the base station, each user""s signal is separated and decoded by first multiplying the total received signal by the complex conjugate of the desired user""s spreading code. This removes the desired user""s spreading code from the received signal or despreads the desired signal back to its original bandwidth and makes other user"" signals look like high bandwidth noise. The despread signal, together with interference due to other user"" signals, i.e. multiple access interference, is used in a conventional CDMA receiver to decode the desired user""s transmitted bits, treating the interference as additive noise. The quality of reception at the base station can be substantially improved if the multiple access interference, rather than being treating as noise, is canceled from the received signal before decoding the desired user""s signal.
To this end, interference cancellation (IC) techniques are employed to try to reduce multi-access interference in a CDMA receiver by estimating the interference due to other users and then subtracting the estimated interference from the received signal before the desired user""s signal is decoded. A multistage or parallel interference canceler (PIC) consists of a number of concatenated stages which are usually identical to one another. The total received signal is passed to the first stage which makes tentative decisions as to the transmitted signals of all the users. While making a tentative decision on a particular user""s signal, all other user"" signals are treated as noise. For each user, an estimate of interference is obtained by respreading and combining the tentative signal decisions of all other users. The interference estimate is then subtracted from the received signal to form a xe2x80x9ccleanerxe2x80x9d signal for that user, which is passed to the next stage of interference cancellation. The next stage uses the cleaner signals for each user to again estimate and subtract interference. This is repeated for any desired number of stages with two to four stages being typical. Output signals from the final stage are used by a conventional CDMA decoder to make symbol decisions, i.e., to determine what symbols were in the received signal.
At each mobile telephone, bits of the user""s signal can be modulated for example as binary phase-shift keying (BPSK) signals or as M-ary orthogonal signals (as in IS-95 North America CDMA standard) prior to spreading. With BPSK modulation, the baseband signal of a user takes the values +1 or xe2x88x921 depending on whether the bit is a 0 or a 1. With M-ary orthogonal modulation, a group of log2 M bits are mapped onto one of M Walsh codes, each Walsh code having M bits taking values xe2x88x921 or +1. For example, M=64 in the uplink of IS-95 CDMA standard, so that 6 bits are modulated to one of 64 Walsh codes with each Walsh code being 64 bits long. All M codes are orthogonal to each other. Decoding a BPSK modulated signal after despreading involves integrating over the bit interval and hardlimiting the result. For M-ary orthogonal modulation, decoding is done by computing the correlations of the despread signal with all the M possible Walsh codes and determining the strongest among them.
FIGS. 1 and 2 illustrate a prior art parallel interference cancellation (PIC) arrangement for an IS-95-like CDMA system using M-ary orthogonal modulation with Walsh-Hadamard functions as symbol waveforms. FIG. 1 schematically shows a general architecture of an N-stage PIC 100. Carrier is removed from the received signal to obtain the complex baseband received signal r, which is the sum of all signals received from the K simultaneous telephones or users plus noise.
The output of each stage 102, 104, 106 of the PIC 100 is a set of estimates of all the user"" received signals: u1,n, u2,n, . . . , uk,n, where lower case n is used to indicate the number of any stage and lower case k is used to indicate the number of any user. As shown in FIG. 2, signal uk,nxe2x88x921, which consists of user k""s received signal plus an interference component, is used by the nth stage to reconstruct user k""s received signal. One of K conventional decoders 108, 110, 112, coherent or non-coherent, for M-ary orthogonal CDMA signals are used to decide which one of the M Walsh functions or symbols was transmitted by the kth user. The one of decoders 108, 110, 112 also performs the despreading operation by multiplying the input signal with the complex conjugate of the kth user""s PN code.
The M-ary decoders 108, 110, 112 are followed by Walsh code generators 114, 116, 118 which produce the corresponding symbol waveforms so that a single symbol waveform is used for reconstruction of each symbol waveform. The reconstructed symbol waveform for the kth user is then respread by multiplying it with the kth user""s PN code and scaled by the complex valued channel estimate xc3xa2k to obtain the reconstructed user k""s baseband received signal. For the kth user, the interfering signals from all other users thus reconstructed are subtracted from the total received signal r to produce uk,n. If the symbol decisions in the nth stage are sufficiently accurate, uk,n will have a lower interference component than uk,nxe2x88x921. In general, the amount of interference reduced in the nth stage will depend on the correctness of symbol decisions in that stage.
The received baseband signal r is given to all the inputs for the first stage. The outputs of the Nth stage are used by conventional M-ary decoders 120, 122, 124 such as the ones described above to make final symbol decisions for each user. If the PIC 100 is followed by an error correcting decoder, such as a Viterbi decoder, for channel encoded data, the M-ary decoder may provide soft decisions instead of hard symbol decisions.
The present invention comprises a multistage or parallel interference cancellation (PIC) arrangement for use in a communications system using code division multiple access (CDMA) with M-ary orthogonal modulation, such as the uplink specified by the IS-95 North American CDMA standard. All the M symbol waveforms, weighted according to their correlations with a despread signal, are combined to form a reconstructed symbol waveform for each user in each stage. The reconstructed symbol waveforms for all users are summed to form an estimated composite received signal having reduced interference which is subtracted from the composite received signal to generate a residual interference signal (RIS). While the total received signal is despread in the first stage of the PIC, the RIS or cancellation residue signal from the immediately preceding stage is used in succeeding stages of the PIC. A final decision is made by despreading the RIS from the final stage of the PIC, correlating it with the M symbol waveforms using a Fast Hadamard Transform (FHT). A weight vector from the final stage is then added to the output of the FHT. The real part or the squared magnitude of the resulting correlations, depending, respectively, on whether coherent or noncoherent demodulation is used, is then taken. The final symbol decision is made by selecting the symbol corresponding to the maximum component of the resulting decision statistic vector.
In accordance with one aspect of the present invention, a method of decoding a spread spectrum composite signal, the composite signal comprising a plurality of user signals that have been modulated using M-ary orthogonal signals and spread with a plurality of respective codes, comprises a plurality of interference cancellation steps performed in succession. The steps are performed in response to an input cancellation residue signal and input weight signals corresponding to preceding estimated correlation signals of the users to the M-ary orthogonal signals with each successive step comprising the performance for each one of the user signals of a series of substeps. A despread input cancellation residue signal is generated by despreading the input cancellation residue signal. The despread input cancellation residue signal is correlated with the M-ary orthogonal signals to generate a first correlation signal. The first correlation signal is added to an input weight signal for the user to generate a second correlation signal which is operated on with a decision function to generate an output weight signal for the user. An inverse transform of the output weight signal for the user is performed to reconstruct the user signal. The user signal is spread to generate an estimate of a portion of the composite signal for the user.
Preferably, the M-ary orthogonal signals are M Walsh functions and the step of correlating the despread input cancellation residue signal with the M-ary orthogonal signals to generate a first correlation signal comprises the step of taking the Fast Hadamard Transform (FHT) of the despread input cancellation residue signal. Where the FHT is used, the step of performing an inverse transform of the output weight signal for the user to reconstruct the user signal comprises the step of taking the Inverse Fast Hadamard Transform of the output weight signal for the user. For the first interference cancellation step, the residue signal comprises a received signal and the input weight signals comprise zero signals. Preferably, the step of operating on the second correlation signal with a decision function to generate an output weight signal for the user comprises the step of operating on the second correlation signal with a sigmoid decision function or linear decision function.
In accordance with another aspect of the present invention, a method for interference cancellation from a received spread spectrum composite signal, the received composite signal comprising a plurality of user signals that have been modulated using M-ary orthogonal signals and spread with a plurality of respective spreading codes, comprises despreading the received composite signal with a plurality of respective despreading codes corresponding to the plurality of respective spreading codes to form a despread composite signal for each user. The despread composite signal for each user is correlated with the M-ary orthogonal signals to generate a correlation signal for each user. The correlation signal for each user is operated on using a decision function to generate a weight signal for each user. An estimated received signal is reconstructed for each user from the weight signal, and the estimated received signals for all of the users are combined to generate an estimated received composite signal.
The method may further comprise subtracting the estimated received composite signal from the received composite signal to generate a cancellation residue signal and despreading the cancellation residue signal with a plurality of respective despreading codes corresponding to the plurality of respective spreading codes to form a despread cancellation residue signal for each user. The despread cancellation residue signal for each user is then correlated with the M-ary orthogonal signals to generate a first correlation signal for each user. The first correlation signal for each user is added to the weight signal for each user to generate a second correlation signal for each user. The second correlation signal for each user is operated on with a decision function to generate an updated weight signal for each user. An updated estimated received signal is reconstructed for each user from the updated weight signal for each user, and the updated estimated received signals for all of the users are combined to generate an updated estimated received composite signal. The method may further comprise step of subtracting the updated estimated received composite signal from the received composite signal to generate an updated cancellation residue signal.
In accordance with yet another aspect of the present invention, apparatus for decoding a spread spectrum composite signal, the composite signal comprising a plurality of user signals that have been modulated using M-ary orthogonal signals and spread with a plurality of respective codes, comprises a plurality of interference cancellation stages. Each of the stages is responsive to an input cancellation residue signal and input weight signals corresponding to preceding estimated correlation signals of the users to the M-ary orthogonal signals. Each stage comprises for each of the user signals means for despreading the input cancellation residue signal to generate a despread input cancellation residue signal. Means provide for correlating the despread input cancellation residue signal with the M-ary orthogonal signals to generate a first correlation signal. Means provide for adding the first correlation signal to an input weight signal for the user to generate a second correlation signal. Means provide for operating on the second correlation signal with a decision function to generate an output weight signal for the user. Means provide for performing an inverse transform of the output weight signal for the user to reconstruct the user signal. And, means provide for spreading the user signal to generate an estimate of a portion of the composite signal for the user.
In accordance with still another aspect of the present invention, apparatus for interference cancellation from a received spread spectrum composite signal, the received composite signal comprising a plurality of user signals that have been modulated using M-ary orthogonal signals and spread with a plurality of respective spreading codes, comprises means for despreading the received composite signal with a plurality of respective despreading codes corresponding to the plurality of respective spreading codes to form a despread composite signal for each user. Means provide for correlating the despread composite signal for each user with the M-ary orthogonal signals to generate a correlation signal for each user. Means provide for operating on the correlation signal for each user with a decision function to generate a weight signal for each user. Means provide for reconstructing an estimated received signal for each user from the weight signal. And, means provide for combining the estimated received signals for all of the users to generate an estimated received composite signal.
It is, thus, an object of the present invention to provide an improved multistage interference canceler for CDMA communications systems using M-ary orthogonal modulation; to provide an improved multistage interference canceler for CDMA communications systems using M-ary orthogonal modulation wherein all M symbol waveforms, weighted according to their correlations with a despread signal, are combined to form a reconstructed symbol waveform for each user in each stage; and, to provide an improved multistage interference canceler for CDMA communications systems using M-ary orthogonal modulation wherein a cancellation residue and individual weight vectors of all users are used for interference cancellation.
Other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims.